DIY_EFI Digest Wednesday, 6 March 1996 Volume 01 : Number 067 In this issue: Re: MC3334 question Re: Fuel Maps See the end of the digest for information on subscribing to the DIY_EFI or DIY_EFI-Digest mailing lists. ---------------------------------------------------------------------- From: dn Date: Tue, 5 Mar 96 22:49:57 MDT Subject: Re: MC3334 question > Michael Fawke wrote: > Pardon my ignorance, but I am a computer programmer and my knowlege of > analogue is _very_ limited.... Ya, I feel sorry for you electrically challenged individuals... (just kidding! 8-) > On the MC3334, when used to drive an MJ10012 (?) with current sense, the > circuit diagram I have shows a .027 Ohm resistor bewteen ground and the > MJ10012, with the sense taken from the MJ10012 side. In Tim Drury's article > on Distributorless ignition, he gives a circuit for the ignitor which does > not have this resistor. I am assuming the resistance is necessary for the > current sense to work, - so what can be used to create this resistance, or > can it really be discarded (as per Tim Drury's design)? After a little data book browsing, I see MC3334 has an overcurrent limit threshold of minimum 120 mV, typical 160 mV, maximum 190 mV. This means that at the current level you want to limit at, the voltage across the current sense resistor has to rise to around 160 mV to trip the current limiter circuit. In the data book, they show a circuit with a 0.075 ohm resistor, and a further adjustable voltage divider into pin 8 (the current limit input). This circuit is setup to limit at 5.5 A. If you multiply 5.5 by .075 ohm, you get 412 mV across the resistor Rs. This is divided further by a resistive divider circuit, comprised of RD1 and RD2. They give a formula in the data sheet for calculating RD1 and RD2 for the desired current limit. Tim Drury may not have used the current limit, but I wouldn't recommend it unless you use a coil which requires a separate ballast resistor. The different value for Rs in your circuit is 0.027 ohms, which would give a current limit of about 6 A. Your circuit possibly doesn't use the voltage divider shown in my data sheet, which is added to make the current limit adjustable. As far as finding a resistor for Rs, low value power resistors are kind of scarce. You could consider paralleling several smaller resistors to get the required value. Let's see, this resistor would dissipate about 2.5 W @ 6 A (I^2*R). You could parallel 4 X .1 ohm, 1W resistors (or 10 X .27 ohm .5W) to give .025 ohm @xxx. > Also, how do you pick a coil to work with this design? Most coils I have > seen (single output) are sold with no info on resistance. Can the M3334 (when > used with current sense) use _any_ coil? I am currently using a coil with > a ballast resistor and would prefer not to fork out another $50 Australian > for another coil :) Any coil will work, the circuit works by limiting the maximum current into the coil (which keeps it from heating up at low RPM's), which also eliminates the need for the ballast resistor. The ballast resistor ALSO limits current at HIGH RPM's, though, which decreases ignition energy and kindof defeats the purpose... If you decide NOT to use the current limit in the MC3334, then you WOULD need the ballast resistor. To disable the current limit, tie pin 8 to ground. regards, dn - -- - --------------------------------------------------------------------- Darrell A. Norquay Internet: dn@xxx.ca Datalog Technology Inc. Bang: calgary!debug!dlogtech!darrell Calgary, Alberta, Canada Voice: +1 (403) 243-2220 Fax: +1 (403) 243-2872 @ + < __/ "Absolutum Obsoletum" - If it works, it's obsolete -------------------------------------------------------------------- ------------------------------ From: "SANDY" Date: Wed, 6 Mar 1996 11:03:11 UTC-2 Subject: Re: Fuel Maps On 4 March, Sean wrote: > My main question here is: > How does everyone out there tune the fuel (and ignition) maps on their > system? > I would also be interested in the answer to that question. Has anyone used on-board dynos like the DynoLab or Schrick versions and if so, how succesful was it? Or do you run out of road before you finish one RPM/load combination? Sandy ====================================================================== DR. S.M. WELTAN DEPT OF PHYSIOLOGY Tel. No: (021) 406-6507 UNIVERSITY OF CAPE TOWN Fax No: (021) 47-7669 MEDICAL SCHOOL SOUTH AFRICA e-mail:sweltan@xxx.za ====================================================================== ------------------------------ End of DIY_EFI Digest V1 #67 **************************** To subscribe to DIY_EFI-Digest, send the command: subscribe diy_efi-digest in the body of a message to "Majordomo@xxx. A non-digest (direct mail) version of this list is also available; to subscribe to that instead, replace "diy_efi-digest" in the command above with "diy_efi".